Plasma and Fusion Research: Letters

Volume 12, 1305038 (2017)

Hydrogen Permeation through Sputter-Deposited Tungsten
Coated F82H in QUESTYue XU1) , Arseniy KUZMIN2) , Yoshi HIROOKA1,3) , Kazuaki HANADA2) ,Naoko ASHIKAWA1,3) and the QUEST group2)
1)

2)

The Graduate University for Advanced Studies, 322-6 Oroshi, Toki, Gifu 509-5292, Japan
Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 812-8580, Japan
3)
National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan(Received 20 April 2017 / Accepted 14 August 2017)
Hydrogen plasma-driven permeation (PDP) experiments have been performed using a sputter-depositedtungsten (SP-W) coated F82H membrane in the spherical tokamak QUEST. It has been found that SP-W coatings tend to enhance hydrogen PDP compared with that of bare F82H membrane. Surface recombination is a key
process determining the PDP flux, suggesting that surface eﬀects on hydrogen PDP should be further investigated.
c 2017 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: QUEST, first wall, F82H, tungsten coating, plasma-driven permeation
DOI: 10.1585/pfr.12.1305038
In magnetic fusion devices, confinement loss particles
via cross-field diﬀusion will bombard the first wall to induce plasma-driven permeation (PDP) (atomic hydrogen
permeation included). Hydrogen isotopes (deuterium andtritium) transported into the breeding blankets by PDP willhinder the recovery eﬃciency of tritium and necessitateisotopes separation. Tritium permeation through the firstwall may raise reactor safety issues as well. In addition,
particle recycling from the plasma-facing wall would directly aﬀect the core plasma performance [1]. Thus, it is
crucial to measure the particle flux to the first wall. In ourprevious work [2], a permeation probe made of a reduced
activation ferritic steel (RAFS) F82H was used to measure
the hydrogen permeation flux in QUEST at Kyushu University. Those measurements were conducted during wallconditioning steady-state discharges. For a DEMO reactor,
surface coatings made of refractory metals such as tungsten (W) are necessary to protect the plasma-facing wall
made of RAFS such as F82H [3]. The characterization ofhydrogen transport through W coated F82H under plasma
exposure is of crucial importance to evaluate major reactor
design issues including tritium retention, breeding feasibility and first wall particle recycling. In this work, a sputterdeposited tungsten (SP-W) coated F82H permeation probe
has been installed in QUEST. The data have been compared with experimental results obtained by a bare F82H
permeation probe [2] and the eﬀects of SP-W coatings onhydrogen PDP are discussed.
Experiments are performed in the medium sized
spherical tokamak QUEST [4] with the major and minor
radii of 0.68 m and 0.4 m, respectively. Schematic view of
the tokamak cross-section is shown in Fig. 1 (a). The workauthor’s e-mail: xu.yue@nifs.ac.jp

Fig. 1 (a) Schematic view of QUEST cross-section, and (b)schematic diagram of the permeation setup.

ing gas is hydrogen, supplied from a nozzle on the centralstack. Gas puﬀ is done in a pulse mode, the opening timeof which varies in the range of 5 - 100 ms with the fueling rate of 2.0 × 1021 H s−1 . The interval between puﬀs isin the range from 1 s to 50 s. The fueling gas pressure inthe vessel is set in the range of 1 - 10 × 10−3 Pa. Plasma
is produced by electron cyclotron resonance (ECR) alone
at 2.45 GHz (<10 kW). The annular slab plasma is used in1305038-1c 2017 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Letters

Volume 12, 1305038 (2017)

Fig. 2 SEM image of the SP-W coatings surface.

this study, indicated in pink in Fig. 1 (a). The plasma diameter can be adjusted by changing the toroidal magneticfield. The plasma parameters are measured by an array ofLangmuir probes at the “top plate”. In addition, upper andlower oblique plates and vertical plates (radiation shield)
are installed and denoted by grey lines. The vessel temperature is kept at ∼100 ◦ C during experiments.
Shown in Fig. 1 (b) is a schematic diagram of the permeation setup. In the present work, a SP-W coated F82H
sample is used. The permeation area is 35 mm in diameter. The thicknesses of W coatings and F82H substrate are
0.5 µm and 0.25 mm, respectively. The permeation probe
has been installed inside the closed port at the mid-planeand the membrane is ∼15 cm recessed from the radiation
shield. A resistive heater is set behind the membrane tokeep the sample temperature in the range of 300 - 350 ◦ C.
The temperature is measured by a thermocouple attachedto the downstream surface. The sample has been in-situ
degassed at ∼350 ◦ C for 8 h before experiments. Hydrogen
permeation flux is measured by a quadrupole mass spectrometer (QMS) which has been calibrated by a hydrogenstandard leak. Shown in Fig. 2 is the surface morphologyof SP-W coatings. The density of W coatings has been
evaluated from the weight increase and film volume afterW deposition, which is 19.2 g/cm3 , ∼99.5% of bulk polycrystalline W. Characteristic microstructure of SP-W coatings has already been published elsewhere [5].
Hydrogen PDP flux through the SP-W coated F82H
membrane has been measured under the wall conditioning 910 s discharge in QUEST. The hydrogen plasma
is produced by the 2.45 GHz ECR system with an input
power of 7.5 kW and the toroidal filed coil current is kept
at 17 kA. This condition is same with that of the previous work [2]. The steady-state permeation flux for theSP-W coated F82H membrane has been measured to be∼2.1 × 1013 H/cm2 /s at 300 ◦ C.
In our previous work [2], the steady-state permeation
flux was measured to be ∼2.4 × 1013 H/cm2 /s at 270◦ C using a 0.2 mm thick bare F82H membrane. Notice, however,that there was no radiation shield during that QUEST campaign. Therefore, to evaluate the eﬀects of the radiation

Fig. 3 Radial profiles of hydrogen incident flux (Γinc ) by a reciprocate PdCu permeation probe (indicated as PP8 inFig. 1) and ion flux (Γion ) by a reciprocate Langmuirprobe (indicated as RLP in Fig. 1) in QUEST (reedit fromRef. [6]).

shield on hydrogen permeation, some calibration data arenecessary. Shown in Fig. 3 are the radial profiles of hydrogen incident flux (Γinc ) measured by a reciprocate PdCu
permeation probe (indicated as PP8 in Fig. 1 (a)) and ionflux (Γion ) measured by a reciprocate Langmuir probe (indicated as RLP in Fig. 1 (a)) [6]. The PdCu membrane isselected for the probe calibration system from favorable
viewpoints including the faster response time, wider permeation regime, and higher sensitivity independent of theincident hydrogen flux [6]. The characteristics of the PdCu
permeation probe have already been experimentally investigated by Onaka et al. [7]. It has been found that hydrogen
permeation through PdCu membrane is limited by the surface recombination process, in which case the steady-state
permeation flux is proportional to the implantation (incident) flux.
In QUEST, two regions, i.e., the plasma region and inside the closed port section, are separated by the radiation
shield at R = 1.3 m (as shown in Fig. 3). For R < 1.3 m,both Γinc and Γion are smoothly increasing towards the slabplasma. The radial scale lengths and the relative radial profiles of Γinc and Γion are consistent with each other, whilea factor of 4 - 5 diﬀerence is shown. It should be notedthat these radial profiles inside the plasma side reflect theradially diﬀusing plasma profile as well. There are steep
gradients in both Γinc and Γion near R = 1.3 m, indicating that the diﬀusing plasma is terminated by the radiation
shield around the torus. For R > 1.3 m, the Γinc profile

1305038-2

Plasma and Fusion Research: Letters

Volume 12, 1305038 (2017)is almost constant, ∼0.2 × 1019 H/m2 /s, while Γion immediately becomes zero. Thus, it can be concluded the ion
flux is dominate in the plasma region (R < 1.3 m) but theatomic hydrogen plays an essential role on the permeation
behind the radiation shield or inside the closed port section(R > 1.3 m).
In the present work, the permeation membrane is
15 cm recessed from the radiation shield. In the previouswork, however, the membrane is 3.5 cm away from the outboard wall, the positions of the probes are shown in Fig. 3.
It can be seen that without the radiation shield, the hydrogen incident flux Γinc measured by the PdCu probe is extrapolated to be ∼0.8 × 1019 H/m2 /s at R = 1.345 m, whilewith the radiation shield, Γinc at R = 1.345 is reduced by afactor of ∼4 to be the value of ∼0.2 × 1019 H/m2 /s. For
hydrogen PDP taken in the RD regime (recombinationlimited hydrogen release from the upstream surface and thediﬀusion-limited hydrogen release from the downstreamsurface) [2], the permeation flux J+ (atoms/cm2 /s) is proportional to the square-root of implantation (incident) flux
J0 (atoms/cm2 /s) and inversely proportional to the membrane thickness L (cm):
D J0,(1)
J+ =L Kr
where D (cm2 /s) is the diﬀusion coeﬃcient. Kr (cm4 /s)is the hydrogen recombination coeﬃcient on the plasmafacing surface.
To the first order of approximation, the membrane
temperature (300◦ C in this work and 270◦ C in Ref. [2]) effects on D is negligible [8]. The membrane temperatureeﬀects on J+ for SP-W coated F82H has been shown inRef. [5], J+ has been found to increase with decreasing thetemperature, which means that J+ is supposed to be evenhigher than the present measured value for the membrane
temperature at 270◦ C during experiments. Therefore, from
Eq. (1) one obtains that the permeation flux will be reduced
at least by a factor of ∼2.5 taking into account the eﬀectsof the incident flux and membrane thickness. The steadystate hydrogen permeation flux has been re-estimated to be∼5.3 × 1013 H/cm2 /s at 300◦ C in the present work.
Shown in Fig. 4 is a comparison of hydrogen PDP
through the SP-W coated F82H and bare F82H membranes. The normalized PDP flux means that the diﬀerent
incident flux due to the radiation shield and the diﬀerent
membrane thickness have been considered so as to compare with the data in Ref. [2]. It can be seen that the permeation flux for bare F82H reaches steady-state within 100 s,
while it take ∼700 s for SP-W coated F82H. It is believed
to be attributed to the trapping eﬀects of W coatings onhydrogen migration which have already been under investigation and the details will be published elsewhere. The
steady-state permeation flux through SP-W coated F82H in
QUEST is about a factor of 2 higher than that through bare
F82H, i.e., SP-W coatings enhance hydrogen PDP, whichis in agreement with the data taken in the linear plasma fa-Fig. 4 Comparison of hydrogen PDP through the SP-W coated
F82H and bare F82H membranes in a 910 s steady-state
wall conditioning discharge. The thicknesses of SP-W
coatings and F82H substrate are 0.5 µm and 0.25 mm, respectively.

cility VEHICLE-1 except that the increase in permeation
flux is relatively smaller. In VEHICLE-1, the permeation
flux through SP-W coated F82H is about one order of magnitude higher than that through bare F82H in the temperature range of 300 - 550◦ C [5]. The diﬀerence could be dueto the diﬀerent particle bombarding energies in QUEST(no DC bias) and VEHICLE-1 (DC bias, −100 V). Besides, the permeation flux measured in QUEST is mainly
a result of atomic hydrogen permeation, which is diﬀerent
from the plasma condition in VEHICLE-1. Nevertheless,further investigations are still needed to address this issue.
In this work, hydrogen permeation measurements
have been conducted in the QUEST spherical tokamak
for SP-W coated F82H membrane and the data have been
compared with our previous results for bare F82H. A much
longer breakthrough time and higher steady-state permeation flux have been observed for SP-W coated F82H. It is
concluded that SP-W coatings enhance hydrogen PDP not
only in the VEHICLE-1 linear plasma facility but also inthe QUEST spherical tokamak.
This research is partly supported by the JSPSNRF-NSFC A3 Foresight Program in the field ofPlasma Physics (NSFC: No.11261140328, NRF: No.
2012K2A2A6000443).
[1] J. Roth et al., J. Nucl. Mater. 390, 1 (2009).
[2] H. Zhou et al., Plasma Fusion Res. 9, 340541 (2014).
[3] T. Otsuka et al., J. Nucl. Mater. 438, S1048 (2013).
[4] K. Hanada et al., Plasma Sci. Technol. 13, 307 (2011).
[5] Y. Xu et al., Plasma Fusion Res. 12, 1205009 (2017).
[6] A. Kuzmin et al., Nucl. Mater. Energy, in press, DOI:10.1016/j.nme.2017.03.027 (2017).
[7] M. Onaka et al., Nucl. Mater. Energy 9, 104 (2016).
[8] Y. Xu et al., Fusion Eng. Des. in press, DOI:
10.1016/j.fusengdes.2017.04.022 (2017).

1305038-3